The Membrane M Protein of the Transmissible Gastroenteritis Coronavirus Binds to the Internal Core through the Carboxy-Terminus DAVID ESCORS, JAVIER ORTEGO, AND LUIS ENJUANES 1 I Department of Molecular and Cell Biology, Centro Nacional de Biotecnologia, CSIC, Campus Universidad Autonoma, 28049 Madrid, Spain. 1. INTRODUCTION An internal core was recently described in two coronavlruses, the tramsmissible gastroenteritis coronavirus (TGEV) and the murine hepatitis virus (MHV) (Risco et aI., 1996). The core has a spherical and possibly icosaedral shape. Purified cores included both the nucleoprotein and the membrane protein, as well as the viral genome. The presence of the M protein in purified cores was unexpected since this has been considered an integral membrane protein. This observation could be explained if the M protein molecules embedded in the viral envelope, with the intravirion carboxy-terminus, interact with the internal core by an intravirion domain. This hypothesis seems feasible since it was shown that the M protein interacts with the viral nucleocapsids in MHV virions (Sturman et aI., 1980). However, further evidence for this interaction is required. The Nidoyiruses (Coronayiruses and Arteriyirusesl. Edited by Ehud LaYI et af .. Kluwer Academic/Plenum Publishers. 200 I. 589